A target substance subject to exhaust gas purification is a particulate matter such as nitrogen oxide, carbon monoxide, unburned hydrocarbon, soot and the like. Various apparatuses for purifying these substances have been conventionally developed.
As an apparatus for reducing the nitrogen oxide (NOx), a denitration apparatus or the like has come into practical use, in which a reduction catalyst using an ammonia or an urea as a reducing agent is installed in an exhaust passage, thereby selectively reducing the nitrogen oxide. Further, in a comparatively compact gas engine or an automotive gasoline engine, there has been developed a three-way catalyst which can simultaneously decompose three elements comprising the nitrogen oxide, the carbon monoxide (CO) and the unburned hydrocarbon (HC), and the three-way catalyst contributes to an effective purification of the exhaust gas.
However, it has been known that the three-way catalyst effectively achieves a purifying operation in the case that the three-way catalyst is operated at a theoretical air fuel ratio or within a range close thereto, but is not effectively operated under the other conditions, particularly in exhaust gas in which air (oxygen) is excess. In order to cope with this, in the gas or gasoline engine operated in the excess air state, a nitrogen oxide occluding catalyst system has come into practical use, which temporarily occludes the nitrogen oxide in an occluding agent at a time of being operated under the excess air (oxygen) condition, and next discharges and reduces the occluded nitrogen oxide by being operated under the excess fuel condition.
However, there has been known that in the nitrogen oxide occluding catalyst system, the catalyst is poisoned by sulfur oxide (SOx) in the exhaust gas derived from a sulfur component in a fuel, and a purifying capacity of the nitrogen oxide is rapidly reduced, and the nitrogen oxide occluding catalyst system is used only in the engine using a low sulfur containing fuel in an actual condition. There has been developed a purifier (Patent Document 1) having a structure which reduces nitrogen oxide adsorbed to the occluding substance and discharging the sulfur oxide or the like, by occluding the nitrogen oxide using the occluding substance in a nitrogen oxide purifying tower and burning the nitrogen oxide in the nitrogen oxide purifying tower. However, because the structure is made such as to burn within the purifying tower having the occluding substance built-in, a durability of the occluding substance becomes actually problematic.
For removing a particulate substance such as the soot, an electric precipitator and a DPF come into practical use. The DPF is structured such as to capture the particulate substance by a filter, and burns and removes the captured particulate substance by an electric heater or the like. There has been recently developed a DPF in which a catalyst component having an oxidizing operation is carried in a fine particle filter, and the particulate substance can be continuously removed.
Patent Document 1: Japanese Patent Publication Laid-Open No. 2003-27927